PROJECT SUMMARY As one of two extant jawless vertebrates, lampreys occupy a pivotal position for study of the evolution of our adaptive immune system. The proposed studies build on previous findings indicating that interactive B- and T-like lymphocytes are fundamental features of the adaptive immune system in both jawless and jawed vertebrates, although jawless vertebrates generate variable lymphocyte receptors (VLR) for antigen recognition by combinatorial conversion of incomplete VLRA, VLRB and VLRC germ-line genes into fully assembled VLR genes using neighboring leucine-rich repeat (LRR) sequences as templates. Prior studies indicate that whereas VLRA & VLRC gene assemblies coincide with expression of cytidine deaminase 1 (CDA1) in the thymus equivalent gill region, concurrent VLRB and CDA2 expression occurs primarily in hematopoietic tissues. A comprehensive analysis is proposed to elucidate the development, distribution and function of the VLRB B- like cells and their antibody products in lampreys. The first specific aim is to elucidate the generation, diversification and expression of the VLRB antibody repertoire, with special emphasis on the role of newly identified CDA2 isoforms. The second specific aim is to define the composition and functional capabilities of three key receptors of lamprey B-like lymphocytes, namely the composite VLRB receptor, IL-17D receptor and BAFF/APRIL receptors, through the use of complementary and novel strategies to amplify the assessable VLRB lymphocyte population. The third specific aim is to generate specific VLRB antibodies against human tumor cell antigens and to modify them for diagnostic and potential therapeutic uses. The overall goals of these studies are to better understand basic aspects of B cell biology in a jawless vertebrate, in particular the mechanism of generating VLRB antibody diversity, and to exploit the advantages of lamprey antibodies for biomedical purposes.